לזכות ידי"נ האהוב ר' אהרן ישראל בן ר' משה מרדכי
We already saw the Rav Yosef Engel in the Asvan Di-oraisa who says that the reason that even a מתעסק is chayav for חלבים ועריות שכן נהנה is because when one is מתעסק it is as if no מעשה is being done and therefore the act is detached from the one doing the act. But with respect to חלבים and עריות no מעשה is necessary since the איסור is the הנאה. Therefore מתעסק is חייב with regard to חלבים ועריות because the issur is the pleasure - שכן נהנה!! Reb Elchonon and others concur.
The Steipler [Kehillos Yaakov Shabbos 35] asks that if this is true that when one is מתעסק there is no מעשה [and he is חייב because of the הנאה alone] - then how can there be a חיוב חטאת for these two transgressions?? There can be no חיוב חטאת if there is no מעשה as we see from one who neglects to perform bris milah or bring his korban pesach for even though he incurs כרת במזיד if done בשוגג there is no חיוב חטאת. Why? Because there is no מעשה. So how can there be a חיוב חטאת for חלבים ועריות?
So said the Steipler: Yes, when one is מתעסק it can't be considered that he performed a מעשה. However, regarding חלבים ועריות the הנאה [which is the איסור] transforms the מתעסק into a מעשה. In Flatbush [I once learned there for six weeks so I know] they would say "Der hana'a shaft ah myseh!!" Now that we have a מעשה we can also have a חיוב חטאת.
Not so fast....
But how then do we understand the opinion of Tosfos [Sanhedrin 62] that when one is נהנה from מלאכת שבת he is חייב. On Shabbos we certainly can't say that the איסור is the הנאה as we do regarding חלבים ועריות. The איסור is clearly the עצם גוף המלאכה.
So the Steipler gives it another shot: Really, Rav Y. Engel and Reb Elchonon are not correct [hey - Torah is Toras Emes and you gotta shoot from the hip] and when one is מתעסק he really IS doing a מעשה. However, this מעשה lacks כוונה. I mean, this guy is preoccupied with doing something else and doesn't intend to do this action so he lacks כוונה. In Flatbush-ese - "felt de kavana". When one has has pleasure - that creates kavana. Try eating your mother or wife's delicious [fill in your FAVORITE FOOD] and not notice you are doing so... You can't. It feels TOO GOOD. The pleasure centers in your brain are going crazy. Dos meint [this means] - the הנאה creates כוונה. That explains why for חלבים and עריות one is חייב even when מתעסק. The מתעסק spells no כוונה which is remedied by the pleasure which creates כוונה [both אכילה and ביאה are pleasure dominated activities]. That also explains the opinion of Tosfos that one is חייב for מלאכת שבת done while מתעסק provided that he had הנאה.